# Questions tagged [ohms-law]

A well known relationship that describes the relationship between Voltage and current through a device's resistance expressed mathematically as V= IR. This formula says that voltage across the device is equal to the current through the device multiplied by the resistance.

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### How to show that it is not possible to exceed the power rating of a 1/4 watt resistor of resistance greater than 1k?

I am currently studying The Art of Electronics, third edition, by Horowitz and Hill. Exercise 1.5 says the following: C. Power in resistors The power dissipated by a resistor (or any other device) is ...
159 views

### Finding current through LEDs in series-parallel circuit

I'm stuck confirming that the current pulled in a circuit will be 15mA. If I have hypothetical LEDs that pull 15mA each from 1.8V, and they are put in parallel (I know this is not a good idea, but ...
37 views

### Calculate current through a resistor attached to an ADC?

I have a resistor, R1 = 100 kΩ, connected between a: DAC PIN_1 outputting 1.34 V ADC PIN_2 At PIN_2 I measure 0.57 V. Can i model the circuit as:PIN_1 -- 100 kΩ R1 -- PIN_2 -- R_ADC -- GND? For V1 ...
109 views

### How is the current through this resistor not obeying ohm's law?

In this video, https://www.youtube.com/watch?v=ks_VShSgPns, a 220 ohm resistor is connected across a 5V DC source (the source happens to be oscillating between 0V and 5V but that's besides the point). ...
105 views

### Is resistance the derivative dV/dI or the ratio V/I?

I'm trying to understand the justification for turning off independent sources when calculating the equivalent resistance in Thévenin's theorem. This answer explains has a good explanation but it ...
59 views

### Reduce DC power supply for battery charging

I have a DC power supply from either a WIFI router or printer (not sure) rated for 13.5V @ 1.2A output. This is a replacement charger for an electric ride on car as it was originally 6V and I've ...
5k views

### Why does voltage drop across resistance contradict Ohm's law?

Very basic question here. I have been learning a ton about Ohm's law, and I don’t seem to ever be able to completely understand it. One of which I am getting particularly confused about is voltage ...
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### Where does Ohm's Law, Kirchhoff circuit laws, etc, come from? [closed]

I know that these various laws that we use are often commonly accepted in practice but I am always curious how far down the rabbit hole you can do until you hit something that you just accept as true. ...
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### How is the current adjusted from a power bank with adjustable current?

When using a power bank with adjustable current I'm just wondering how the current is adjusted? it can't be anything to do with resistance of the recieving component because this would be constant, ...
34 views

### Battery Charger in Parallel with System Load Current Sharing - Power Path Control

Currently, I'm building a project which can either run connected over USB or via a battery. The project itself switches between either high current draw at around 450mA or low current draw at 50mA. ...
75 views

### How to compute the resistance and voltage of this schematic?

I have the following diagram I've set up in a simulator: However I have no idea how they get 3.75V at that point (the upper left spot is a voltage reader) nor do I really even know how to apply Ohm's ...
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### Need assistance to understand Power and Ohm's Law

Hello I'm looking at a light bar on a vehicle and the specifications say it draws 7.5A at 9-32V and the light bar says it has 36 x 5 Watt LED lights which would make a total of 180 Watts that this ...
103 views

### Input voltage control by resistor, is this method is correct or not?

In my application I have a requirement to switch 40V to 5V with the help of a linear voltage regulator. My load circuit consumes approx 7 to 8 mA. Due to the unavailability of any specific model of a ...
129 views

### Calculate voltage and amp drop of resistance

How do i calculate the voltage and amperage drop from resistance on a DC series circuit? I know ohm's law and all, but I’m doubting how much of actually applies to this equation, since i know the ...
91 views

### Help in understanding a IR repeater schematic

I'm relatively new to electronics projects, as a hobby. So please bear with me. I want to build an Infrared Repeater as a small project. I've found a component which I think will work nicely for the ...
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### Possible contradiction between Ohm's Law, KCL, KVL and Faraday's Law of Induction?

Let's say, we have a loop of wire of length $L>0$. Let's say, hypothetically, it is a loop of non-ideal wire. That means that the wire has a resistance per unit length $ρ>0$. Let's further ...
218 views

### Why does the voltage drop across an LED matter when determining an appropriate resistor value?

In determining the voltage dropping resistor, why is the forward voltage of an LED deducted from the supply voltage instead of considering the supply voltage alone?
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### Proving the formula for parallel resistors

I am currently studying The Art of Electronics, third edition, by Horowitz and Hill. Exercise 1.3 asks to prove the formulas for series and parallel resistors. I am given the following relevant ...
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### How can these two motors produce the same torque if their volt/amp ratings are different and their watts/RPM are the same

My rudimentary understanding of a DC brushed motor is speed is proportional to voltage and torque is proportional current. Therefore a 36 volt 500 watt motor should have a higher RPM rating and a ...
1k views

### Motor current calculations?

So i've got a question about electrical motors. I put 5V through a small motor and my lab bench PS reads 150mA. If I calculate this using ohms law, my motor resistance should be approximately 33Ω, but ...
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### How exactly does voltage drop across a component?

I have been doing some thought experiment and came across this interesting problem. Say I have a standard normal wire that is 1 meter long, now I have a DC battery at some voltage level and I connect ...
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### How much resistor does an LED lamp need?

I want to make a lamp with LEDs, those have to be powered from a socket (230V). I have a cord with a switch and two wires (ground, voltage), which supports up to 1300 W. The switch for turning it on/...
255 views

### Ohms law's statement

Is this correct: "Current is directly proportional to voltage if resistance kept constant" V = I*R Ohms law is only applied when resistance kept constant or not?
1k views

### What is the max current from a 9 V battery?

Let's take, for example, a 9 V battery. Forgetting about internal resistance or any temperature restrictions, what is the maximum current I can draw from this? Using Ohm's law with a 1 Ω load, this ...
28 views

### Can I compute resistive device power consumption at specific voltage knowing its power rated at some other voltage?

Suppose I have a device containing a heating element and no thermostat such as some simple water heater. It's rated "3500 watts 220 volts" which I assume means that if it's plugged into a properly ...
3k views

### Understanding voltage drop

Meanwhile in quarantine I been trying to learn about electronics. I am still not able to understand the voltage drop when it comes to long wires. My goal is to be able to send a 5v signal over a very ...
91 views

### Can I use two LED in series without a resistor?

I need to put quite a few LEDs on a PCB, and I want to reduce the number of components. The LED I want to use is NCD0603G1, which has a forward voltage of 2.6V to 3.6V (documented.) I have a 5V ...
461 views

### I thought I understood the basics of DC circuits and Ohm's law but now I'm confused

So I was taught in school and also read a lot of articles of the basics of DC circuits. I know the formula R=V/I, and therefore that resistance is directly proportional to voltage and inversely ...
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### Basic question on Ohms law and current

I read that current is 'drawn' and voltage is 'taken'. Im learning basic circuits with an Arduino, and in one tutorial, a resistor is needed to take a 9v power supply and bring it to 30 milliamps for ...
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### Ohm's law and the voltage drop across a resistor

I'm trying to wrap my head around Ohm's law and there are plenty of similar questions already asked in this forum. But I just still don't understand it. It's about calculating resistor values. Let's ...
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### Does voltage drop limit the maximum current that can be draw?

I'm a newbie to this area. I'm having a few questions about the ohm law. The given scenario is, In the circuit, there is a power source of 5V, and a resistor of 1 ohm. This means there will be 5A ...
407 views

### Confusion with Ohm's law

When I connect a cell with 100 Ω resistor. voltage across circuit is 1.4 V and current is 14 mA. That means total power of cell is 1.41 x 0.014 = around 20 mW. Right? Now if I connect two cells ...
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### Calculating the resistor needed for a simple LED and battery circuit

I'm having trouble learning the most basic circuit math! I've a 3V battery pack, two AAA, a standard plastic LED and a pack of resistors and I need to work out which resistor I need. I'm assuming ...
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### Experiment - measuring current across two resistors

My goal is to measure current at different points in the circuit. I took breadboard, D battery, two resistors and multimeter. Here is how my circuit looks schematically: Using multimeter I checked: D ...
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### Ohms law in solar panel

When 2 solar panels are connected in series, the output voltage is sum of both panels but the output current (measured by short circuiting) is the same as single panel. What I dont understand is that ...
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### Generalized Power Equation, Ohm's Law and Ohm's Power Loss?

I'll try to state my questions precisely as I can. I'm a beginner in Electronics and since I have a background in Physical Chemistry and basic Calculus, the book Practical Electronis for Inventors, ...
67 views

### How do you set amperage in a simple circuit?

Volts = Amps * Ohms Theoretically I wanted this (rounded): 3 V = 0.002 A * 1440 Ohms Realistically I got this because a limit of resistors I had (rounded): 3.206 Volts = 0.002 Amps * 1421 Ohms From ...
2k views

### Why does current only choose path of least resistance sometimes

Say we have the following setup simulate this circuit – Schematic created using CircuitLab Input is pulled high and current flows through it as long as the switch is open. As soon as it closes ...
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### Current in a closed DC circuit

I am in the process of learning the fundamentals of electricity, however I am having trouble understanding why there is no measurable current in a closed DC circuit when there is no power consumption. ...
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### Two of the same resistors have different resistance?

I am doing some work on the electronics of a guitar and in doing so was testing some old circuitry. I noticed that there are two resistors each stamped with ‘223’. When checking the resistance of ...
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### Can we get more current by reducing voltage on a power supply?

This is very basic question but I seem can't find the answer. All out there is theory, but no practical explanation about this. Assume I have a 10V 2A power supply. So in theory it outputs power of ...
149 views

### What is a good resistor value for my circuit?

I'm trying to tap into my car's brake circuit to switch between receiving a wireless camera signal and powering my device. For this purpose, I connected a relay to the circuit via a fuse tap. It ...
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### If Voltage determines current , a transformer regulate current?

Well , a transformer can regulate the voltage , increasing and decreasing it . And , if the voltage increase , the current too . So... a transformer regulates current too right? Sorry if its a ...
69 views

### The best way to wind copper wire around rod for lowest resistance

So I have wound 200m copper wire around a rod which gives me 180 Ω, previously I got only ~80 Ω. Both of them were randomly shuffled and the first attempt was on bigger in diameter rod. How to make ...
1k views

### Why is Voltage and Current inversely proportional with Power, but directly proportional with Resistance?

Ohm's Law gives the relationship of Voltage and Current as follows: V = P/I and V = I * R. In the first equation, Voltage and Current are inversely proportional. In the second equation, Voltage and ...
13k views

### Why does current change when adding in an LED?

In the following diagram: The current is 1 mA from: V = IR R = 1 / 1000 = 1 mA However, when I add in an LED with forward voltage 1V: Why does adding in the ...
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### Solving simple parallel/series without Req

Given the following circuit with three resistors, R1=100Ω, R2=200Ω, R3=300Ω: To solve this using the $R_{eq}$ trick: ...
108 views

### Equations for a simplified transistor circuit

In the following circuit: I would like to greatly, greatly simplify how the components work just to be able to make a dumbed-down version of the equations. Let's assume the forward-voltage of the LED ...